function objToSearchStr(obj){
    let str = '';
    for(let key in obj){
        str += `${key}=${obj[key]}&`;
    }
    return str.slice(0,str.length - 1);
}

function ajax( method,url,params,cb ){
    let xhr = new XMLHttpRequest();
    xhr.open(method,method == 'get' ? url + '?' + objToSearchStr(params) : url);
    xhr.onreadystatechange = function(){
        if(xhr.readyState == 4){
            cb(JSON.parse(xhr.responseText));
        }
    }
    xhr.setRequestHeader('content-type','application/x-www-form-urlencoded');
    xhr.send(method == 'get' ? null : objToSearchStr(params));
}